Zinc bacitracin is a performance-promoting substance which is added to animal fodder as a feed additive to improve the utilization of the fodder and hence increase the daily weight gain. In chemical terms, zinc bacitracin is a cyclic polypeptide which is relatively stable in solid and dissolved form. In feed mixtures zinc bacitracin is substantially less stable, a fact which is blamed partly on the effect of heat and moisture during the pelleting of the fodder and the mechanical stress during the processes of mixing the feed additive with the fodder, and also on reactions with heavy metals present in the fodder, such as copper and iron, and with organic compounds such as ascorbic acid.
Numerous expedients have been tried to overcome the known disadvantages and stabilize zinc bacitracin in feed mixtures. Until now, however, nobody has succeeded in finding a fully satisfactory and practical solution to the problem of the reduction in the zinc bacitracin content under normal and particularly under unfavorable conditions of processing and storage, with the result that instances of accidental overdosing could not be ruled out due to the uncertainty as to the zinc bacitracin content.
German Offenlegungsschrift 2,745,035 describes a process in which zinc bacitracin is precipitated from a culture broth, mixed with calcium carbonate and spray-dried, This method makes use, on the one hand, of the greater stability of the zinc compound with respect to bacitracin, and on the other hand spray-drying ensures that the product has a very low water content. Spray-drying is based on the principle of eliminating water from a formulation. The disadvantage of this process is the fact that the costs are higher than with other drying methods. Moreover, dried products very quickly reabsorb the quantity of water corresponding to the adsorption isotherm, which means that all production and packaging steps subsequent to spray-drying must be carried out under conditions of low moisture.